The invention relates to a circuit breaker, which is in a fault situation arranged to disconnect an electrical apparatus, such as a distribution transformer, from an average voltage network or a high voltage network at each terminal, and which comprises at each phase at least one fixed contact and a moving contact to be engaged with and disengaged from the fixed contact, each moving contact being fastened to a turning shaft of the circuit breaker and the circuit breaker being simultaneously electrically connected in series with a high voltage fuse situated at each phase.
Because of more and more stringent quality requirements of the electric power, the amount of interruptions in the use of a power line should be minimized and their duration shortened as much as possible. When a fault occurs in a distribution transformer, the length of interruption in the use of the line feeding electric current is shortened by immediate automatic disconnection of the transformer from the electrical network, because the line can be held live all the time during fault diagnosis and during preparations for changing the faulty transformer.
Oil-insulated distribution transformers have the special problem that, if a fault occurs in a winding of a transformer, the transformer oil in the transformer tank is heated and a gas mixture is generated in the tank. At its worst, the pressure in the tank rises so high that the tank tears and transformer oil leaks out on the ground causing environmental problems, risk of ground or other fire, or in the worst case, danger of explosion threatening human lives. Maintenance personnel is subjected to a particularly great danger when examining a faulty live transformer.
Distribution transformers and many special transformers are characterised in that their protective devices must operate without auxiliary supply voltages, possibly located outdoors, subjected to severe environmental conditions, for which reason the solutions known in the environment of high voltage transformers and implemented by means of protective relays and circuit breakers cannot be used economically in this connection.
A known solution for possible faults in a distribution transformer is to provide the primary side of the transformer with high voltage fuses. This solution has, however, the drawback that a two- or three-phase transformer remains live in a fault situation, because each phase is protected by its own fuse. After the fuse of one phase of a three-phase transformer has blown, current still flows therein. Additionally, when normal current limiting fuses are used, a coordination problem occurs, meaning a situation in which the current is not disconnected by fuse blow out, but remains flowing and causes a pressure rise inside the fuse until the fuse explodes. Standard IEC 282-1 (1985) does not require a breaking capacity of high voltage fuses at low over-currents (generally below 3*In). Further, for instance a cycle short circuit may cause such a low fault current that it cannot be indicated from the strength of the primary current and especially not protected by means of fuses, but a fault of this kind still causes a temperature and pressure rise in the transformer tank, and local oil heating causes gas generation.
French Patent 2 712 730 discloses a solution in which the primary circuit of a transformer at each phase comprises two high voltage fuses of different types in series with a three-phase circuit breaker. The circuit breaker opens according to the properties of the dielectric liquid of the transformer changing over a preset threshold value. This solution eliminates the coordination problem with fuse protection, but still leaves the transformer live at a reduced number of terminals in a fault situation, if the circuit breaker does not open. In addition, so much space is required for positioning he two fuses connected in series in connection with the transformer that the used standard transformer cannot be replaced as such by a solution of this kind without any alterations.
Among other close applications can be mentioned standard IEC 420 xe2x80x9cHigh Voltage alternating current switch-fuse combinationsxe2x80x9d, which defines the electrical and operative properties of fuse circuit breakers intended for AC use in the average voltage area.
As far as fuse circuit breakers using high voltage fuses as a trigger are concerned, it can be mentioned that, because the requirements for rated current are even dozens of times higher than the different requirements for the application area of the invention, said fuse circuit breakers shall always in practice be provided with complicated mechanisms released by a trip pin of a fuse striker, for increasing the power. For instance, the circuit breaker of a 160 kVA distribution transformer shall have a rated current of 15 A, while the lowest rated currents of circuit breakers are 630 A.
The object of the present invention is to eliminate the above drawbacks. This object is achieved by means of a circuit breaker, which is characterised in that the shaft of the circuit breaker is provided with at least one link-spring mechanism for holding the contacts live in connected position and for pushing them apart to the extreme disconnected position when disconnected, while the shaft is brought over the dead spot of its turning, and that the shaft is at each phase provided with a lever arm, each lever arm being arranged to turn by means of a trip pin of a striker of the high voltage fuse the shaft of the circuit breaker and thus the moving contacts of all phases from said connected position over said dead spot of turning said shaft.
Because the breaker mechanism according to the invention always breaks the voltage of all phases in a fault situation, the windings of the transformer remain entirely without voltage and current, though the fuse of one phase only has blown. The faulty transformer does not warm up then and no explosion sensitive gas mixture is generated, for which reason it is safe to examine the transformer and to change it into a new one.
The problem with the coordination of the fuse is eliminated, because upon the fuse blowing out, the circuit breaker opens and disconnects the current, and an explosion of the fuse is thus avoided. In the solution of the invention, only one fuse is needed for each feeding phase. If the fuse is integrated into a feedthrough insulator, the transformer can be provided with mechanical dimensions which make it changeable for a conventional transformer without protection.
The circuit breaker of the transformer according to the invention is further characterised in that the circuit breaker can be tuned to a working condition at the assembly of the transformer. The release limit of the arrangement can be dimensioned in such a way that the circuit breaker does not open in any switching or overload situation of the transformer, but only if the transformer becomes faulty. The mechanism can thus be retuned to the working condition at the maintenance and repair of the transformer, and no separate outside mechanisms are then needed for the control of the circuit breaker.